net: dsa: add missing dsa_switch mdiobus remove

To prevent memory leakage on unbinding, add missing mdiobus unregister
and unallocation calls.
Reviewed-by: NFlorian Fainelli <f.fainelli@gmail.com>
Signed-off-by: NNeil Armstrong <narmstrong@baylibre.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/dsa/dsa.c

@@ -424,6 +424,8 @@ static void dsa_switch_destroy(struct dsa_switch *ds)
 	if (ds->hwmon_dev)
 		hwmon_device_unregister(ds->hwmon_dev);
 #endif
+	mdiobus_unregister(ds->slave_mii_bus);
+	mdiobus_free(ds->slave_mii_bus);
 }
 
 #ifdef CONFIG_PM_SLEEP